1. Field of the Invention
The present invention relates to hydraulic systems, and more particularly to valve assemblies for controlling the flow of hydraulic fluid to and from an actuator to produce bidirectional motion.
2. Description of the Related Art
Various types of mobile equipment are operated by a hydraulic system that drives an actuator, such as a hydraulic cylinder and piston arrangement, which receives pressurized fluid controlled by a hydraulic valve. A typical four-position control valve selectively applies the pressurized fluid to one of two cylinder chambers and drains the hydraulic fluid from the other chamber, thereby driving the actuator in one of two directions depending upon which chamber receives the pressurized fluid. Usually a proportional control valve is employed, which can be opened to varying degrees to control the rate of fluid flow to and from the associated actuator, thereby moving the element of the machine that is connected to the actuator at different speeds.
Mobile equipment often incorporate auxiliary hydraulic valves for optional or lower usage type functions. A relatively low flow control valve usually acceptable for these auxiliary functions. If electrohydraulic operation is required, simple on/off valve can be used. For example, direct acting solenoids often are utilized to shift conventional spools in a manner similar to that employed in manual valves. On/off cartridge valves also may be utilized for this purpose, but in applications that require a three-position, four-way valve arrangement, cartridge valves become relatively large and complex, so as to not be cost effective.
Conventional three-position spool valves, that are commonly used to control auxiliary functions, have a center or neutral position which blocks the flow of fluid from the pump, as well as blocking the connection of the workports to tank. In hydraulic circuits that provide load sensing to control the supply pressure from the pump, these spool valves also require a bleed connection in the neutral position to relieve the load sense pressure signal. For bidirectional operation, a load sense signal must be provided, regardless of the direction of the valve motion. This is often accomplished with a bridge type connection through which the workport pressure flows in both directions of valve operation.
Thus, it is desirable to duplicate the function of a three-position, four-way control valve with solenoid operated valves in an assembly which is as cost effective as possible.